SKP wrote the paper, conducted background research, and performed HIV sequence analysis. JKW contributed to writing and editing. JDB programmed the evolutionary simulations and contributed to writing and editing.
We wish to thank Drs. John Huelsenbeck and Bill Bruno for helpful discussion and Drs. Carla Kuiken and Brian Foley for technical assistance. This work was supported by grants R01 NS51132 (S.K.P. and J.K.W.), VA Merit Award (J.K.W.), and AI066917 (J.D.B.).